Reading a New York Times article last month about the Dreamliner’s troubles, something jumped out at me. Deep in the article were the words Chicago-based Boeing.
Of course I know that Boeing moved its headquarters to Chicago some years back. But still, airplanes are made at airports, else how will they fly away?
Boeing field, Seattle, where the first B-17 crashed on takeoff with the controls locked. Boeing Field, almost downtown, nestled between the freeways. Boeing Field, where I myself went in 1974 for simulator training on the new B-727-200. Everett, WA, where the mighty Whale has been put together since forever. Boeing and its people are a part of the Pacific Northwest, and nothing will ever change that.
The Dreamliner Project
On the technical front the B-787 is ambitious. Carbon-fiber construction, fly-by-wire and all that, of course, in the service of reduced weight and therefore greater economy of operation. But these technologies have been proven in the field with the B-777 and Airbus aircraft. The leap forward is the almost complete elimination of hydraulic systems, replacing them with electrically operated everything. Six huge (by aircraft standards) generators provide enough power for a small city. And infamously, two lithium-ion battery packs provide power to start the APU and to back up cockpit instruments.
It is on the business front, however, that the real leap has been made. Following business school dogma, or perhaps to be near their largest customer, Boeing management made the move to Chicago. There they hatched a plan to build the new plane by doing less than forty percent of the work. The rest they farmed out to fifty-odd “strategic partners” around the world (See James Surowiecki's Requiem for a Dreamliner?in the February 4, 2013 New Yorker).
The Problem
As everyone now knows, the B-787 has been grounded following two incidents involving the meltdown of a lithium-ion battery pack. My own experience with these batteries is limited to a lightweight portable circular saw I used last year in a roofing project. It worked beautifully, except for when I used it to rip a ten-foot length of decking and overheated its battery pack. Wisely, it shut down and refused to go further. I had to wait half a day for the pack to cool enough to be recharged.
That battery pack still works fine today, thanks to the built in sensors and software. And I have learned to appreciate the lithium-ion battery's tendency to overheat when a large current demand is placed on it, such as ripping a long board or starting an APU.
From the New York Times' coverage this last month I also learned that the lithium-ion battery produces oxygen when it overheats, leading to the possibility of an uncontrollable fire. It would seem that when it comes to using lithium-ion batteries, it would be wise to know what you are doing.
The Solution
Recently Elon Musk (PayPal, Tesla, SpaceX) wrote to Boeing to offer assistance with the battery problem. His offer was declined (Flight Global, January 29, 2013). Apparently, it is better to look like you know what you're doing than to actually know anything. Isolated in Chicago, Boeing management are doing damage control by gagging their Chief Engineer in Seattle and labelling the battery failures “maintenance issues.” (For an elucidation of this process in aviation, see the papers and speeches of Mark H. Goodrich.)
Whither the Dream?
Flying is based on confidence. We fly because it is cheap and we believe it is safe. We are lured by deals and fed statistics proving that flying is safer than it ever has been before. We understand that with today's technology aircraft usually “land themselves.”
But none of this is so.
Our single most polluting act is not driving a car or consuming coal-fired electricity, but taking an airline flight. The cost of climate change is not factored into those cheap tickets. But – perhaps closer to home – neither is the cost of non-existent pilot training or remote project oversight in aircraft manufacture. The statistics are re-assuring but mask obvious truths: a) in two recent tragedies the pilots stalled the airplane and didn't recover b) the Dreamliner is at risk of failure because of the financialization of business.
The B-787, as far as I can see, has the potential to be a winner. There are no technical obstacles that cannot be overcome with a little real work. But the danger lies in the dream, in the concentration on maintaining that flying is risk-free. Because, you see, there was something else that jumped out at me in the New York Times article. A Boeing Chief Engineer was quoted about the difficulties of building an airplane with fifty “strategic partners.” It may be my research, but I have been unable to find that quote since.
You’re at the local airport. You watch landings, because pilots always do. Today, because there is a lot of traffic and you're not pressed for time, you watch for awhile. Twenty, maybe thirty landings go by. What do you see? Fifty percent (this is a flight school strip) touch down halfway down the runway. One or two touch down in the last 1000 feet.
What do you take away from the experience?
Gossip, certainly – if you're standing there with fellow pilots. Comfort, possibly – if you feel you can do better than most of what you're watching. Or perhaps chagrin, if the reverse is true and a recent example of your own work sticks in your craw.
But there is a more important take-away: forming an opinion or evaluation by discerning and comparing.That is the dictionary definition of judgment. And the aviation version of judgment is more practical: if I find myself in this situation, can we do it?
The we in the last paragraph refers to you and your airplane. You learn skills and you memorize your airplane's limitations. You are a team.
The situation is whatever pickle you're going to get into on your next flight. Can I land on a 2000-foot runway?
You look up the Landing Distance Required in your Flight Manual or Pilot Operating Manual. For my N-Model Bonanza I find 1600-2000 feet (no wind, 75°F. or less, 2000 feet pressure altitude or less). So we can do it, right?
Not so fast. The runway at my local airport is just under 4000 feet long. I consistently turn off on the center taxiway, but not without some braking. I have a bit too much speed over the fence and I float too long. So I'm not quite ready for that 2000-foot strip. My airplane is, but I am not.
Here is another clue. My POM also lists Landing Distance Required for a Short Field Landing. Same configuration, but the over-the-fence speed is 5 knots less. Instead of 1600-2000 feet, the required distance is 1200-1400 feet. Add five knots and add five hundred feet! Nope: my energy management – hey, my hands and feet, if you get right down to it – are not good enough yet.
Why not, you may ask? After all, I have been a pilot for 45 years. Well, two things: first, I'm 68 years old; and second, after I retired from airline flying I didn't touch a yoke for six and a half years. So I had to write exams and do a lot of re-learning. Now I'm learning the hands and feet again.
In short, for the moment my airplane is better than I am.
Hours, Experience, and Judgment
How do we discern and compare on the road to developing pilot judgment? First, look at the one or two Bottom Guns who touched down in the last 1000 feet. “Good enough,” they say. I guess so, if their airplane can stop in that distance. Then the fifty percent who touched halfway down. “Plenty of runway left. No sweat.” These guys are like me. Their airplanes are better than they are. It's just a matter of how much better.
What's missing? A path to learning judgment.
Experience is measured in hours. Judgment, theoretically, comes from experience. But it is not automatic. Hours of flight or even hours of practice take you nowhere unless they are accompanied by some discernment and comparison. Neither the Bottom Guns nor the halfway-down-the-runway pilots are safe trying to land on a 2000-foot runway. But do they know that?
It could happen to anyone. This time it happened to be a French airplane with French pilots flying for a French airline.
For two years the “black boxes” (the voice recorder and the DFDR) lay in peace on the floor of the Atlantic Ocean, 13, 000 feet below the waves. For two years there was conjecture, speculation, and (some quite fine) attempts at reconstruction. Then the black boxes surfaced, along with other hard evidence, including the jackscrew from the Trimmable Horizontal Stabilizer.
For months as the Bureau d'Enquêtes et d'Analyses slowly released information, we (and the BEA) put together the tragic and terrifying story. But the story stopped abruptly, the last chapter removed or never written.
Now a French aviation writer, Jean-Pierre Otelli, has published that last chapter independently of Air France, Airbus Industrie, and The BEA. The story ends as we knew it would – badly and sadly – but now we have more grisly detail and less room for denial.
The Bureau d'Enquêtes et d'Analyses is incensed. In a press release on October 13, 2011, (look under News in the sidebar) they claim that the transcription released by Otelli “mentions personal conversations between the crew members that have no bearing on the event, which shows a lack of respect for the memory of the late crew members” (my emphasis). The same day the London Telegraph published an account of the final minutes. The account seems to have been shortened since October 13, and I have been unable to find the original. Those who are interested may add the following after “According to an official report released earlier this year, the last words were from Captain Dubois who said: 'Ten degrees pitch.'”:
But in his new book Mr Otelli asks who will be held responsible 'for this mess'. 'Is it a training problem, fatigue, lack of sleep, or is it due to the fact the pilots are confident that an Airbus can make up for all errors?,' he writes. France's air accident investigation unit, the BEA, reacted angrily to the publication of the book, with a spokesman saying printing the conversation showed a 'lack of respect to the memory of the crew who died'. Air France has denied that its pilots were incompetent, but has since improved training, concentrating on how to fly a plane manually when there is a stall. Both Air France and Airbus are facing manslaughter charges, with a judicial investigation led by Paris judges already under way. A judge has already ordered Air France to pay some £120,000 in compensation to the families of each victim, but this is just a provisional figure which is likely to multiply many times over. THE FINAL MOMENTS Marc Dubois (captain): 'Get your wings horizontal.' David Robert (pilot): 'Level your wings. 'Pierre-Cedric Bonin (pilot): 'That's what I'm trying to do... What the... how is it we are going down like this?'Robert: 'See what you can do with the commands up there, the primaries and so on…Climb climb, climb, climb. 'Bonin: 'But I have been pulling back on the stick all the way for a while. 'Dubois: 'No,no, no, don't climb. 'Robert: 'Ok give me control, give me control.'Dubois: 'Watch out you are pulling up. 'Robert: 'Am I?'Bonin: 'Well you should, we are at 4,000.'As they approach the water, the on-board computer is heard to announce: 'Sink rate. Pull up, pull up, pull up. 'To which Captain Dubois reacts with the words: 'Go on: pull.'Bonin: 'We're pulling, pulling, pulling, pulling.'The crew never discuss the possibility that they are about to crash, instead concentrating on trying to right the plane throughout the final four minutes. Dubois: 'Ten degrees pitch. 'Robert: 'Go back up!…Go back up!…Go back up!… Go back up! 'Bonin: 'But I’ve been going down at maximum level for a while.'Dubois: 'No, No, No!… Don’t go up !… No, No! 'Bonin: 'Go down, then!'Robert: 'Damn it! We’re going to crash. It can’t be true!'Bonin: 'But what’s happening?!'The recording stops.
What we know, briefly, is this: Air France 447 ventured into a line of thunderstorms along the InterTropical Convergence Zone. Four other flights diverted around the storms. In the zone the flight encountered unusually warm temperatures and supercooled water droplets – enough to briefly overwhelm the heaters in all three pitot tubes, denying airspeed information to the Flight Control Computers for long enough to cause them to kick off the autopilot and to degrade the flight controls from Normal Law to Roll Direct/Pitch Alternate Law. Despite the fact that by the book they were too heavy to climb, the pilot flying (First Officer David Robert) zoomed up from 35,000 feet to almost 38,000 feet, dissipating the aircraft's energy and exposing it to coffin corner, where Mach buffet meets stalling speed. With brief lapses he held back pressure on the sidestick for the remainder of the flight.
First the airplane stalled (quit flying because the Angle of Attack was too great). Then, because of the steady back pressure on the sidestick, the autotrim wound the Trimmable Horizontal Stabilizer (more powerful than the elevators) to full nose up. (The THS jackscrew was found in this full nose up condition). By now the aircraft was in a deep stall, falling almost straight down in a near-level attitude.
There is plenty of room for argument about why it happened this way. Many (including David Learmount at Flight Global and myself) have started that discussion. It must continue, because we must know not only why F/O Robert stalled the aircraft, but much more importantly why he didn't know he had stalled it, why he had a totally inaccurate picture of what was happening, and why there was a complete absence of situational awareness on that Flight Deck.
It may look as if I am placing blame solely on F/O Robert. Absolutely not. That would be much too easy. I and others have already written many pages (see AF447 on my blog) trying to piece together all the factors at work in this accident. We will write many more.
As in all accidents, there is a chain of events and decisions which gradually (at first!) reduce maneuvering room. The first of these was Captain Dubois' decision to take crew rest approaching the ITCZ.
But before that came Air France's decision to carry less fuel than the spirit of the regulations requires, by filing the Flight Plan as Rio to Bordeaux, alternate Paris. Even earlier, the brilliant (I am not being ironic or facetious, I admire the man) Bernard Ziegler designed the Airbus to be “pilot-proof” and impossible to stall. However he (or his designers) also left the autotrim functional in Pitch Alternate Law, an oversight I believe should be corrected ASAP. Finally, (and earliest of all) you and I and everyone else who has traveled since Airline Deregulation in 1978 believes or wants to believe in cheap seats.
It could happen to anyone.
Sadly, it has all been foreseen. Recently I read an article which bluntly calls out the forces that led to this accident. It is called The Training Paradox, and was written by pilot, engineer, and lawyer Mark. H. Goodrich. Some of the accidents and incidents he describes stood my hair on end. Unfortunately I cannot provide a link to it. I read it in Position Report, November 2011, Volume VIII Number 3. (This is the magazine of the Retired Airline Pilots of Canada).
The very experienced and knowledgeable Mr. Goodrich shows how the forces of deregulation have derailed traditional career paths and interrupted the passing along of knowledge. As a result, the craft, the trade, the profession if you will of flying is dying a slow death. Neither are regulatory bodies or airline management immune from this decay.
This time it was the French. It is not surprising they are in denial. But it could happen to anyone, and it will.
How many times have you heard Airbus pilots say, “It's not an airplane, it's a video game.”?
In this blog I will explore the fact and fiction in this statement. My objective is not to praise a great airplane or run down a flawed one, but rather to find how to live with a perfectly good one.
For almost a decade I flew the A320 (and the A319 and A321) as Captain and Training Captain. I came to see her not as a video game but as a person with whom I had to deal. I came to appreciate her many sterling qualities and also her weaknesses. Both informed our work together.
Fundamentals
First, she is an airplane like any other. If you accelerate her to Vr and raise the nose, does she not fly? If you provoke her into an Angle of Attack above 16°, does she not stall? Do not the laws of aerodynamics still hold?
These fundamental things will always apply as we work through her many wonders: Fly-by-Wire, Envelope Protection, and Flight Guidance systems. These wonders are what software people call a “front end” to her conventional aircraft qualities. But the wonders can be a powerful distraction as well as a boon.
Another pilot comment heard (more frequently in the first decade of operation, roughly the 1990's) is “What the #*%# is it doing now?”. The question was being asked because the pilots didn't know where to look for the answer, and also because an airplane maneuvering on her own was a novelty. When things happen that we don't understand, we human beings tend to see them as acts of God. We substitute reverence for understanding. A320 software – partly because it is so good, most of the time – has been an object of such reverence.
But just as we are not perfect, neither is this marvellous software. As the history of the airplane in service demonstrates, it is only as good as its interface with the pilots.
The Crashes
The first three crashes – Mulhouse, Bangalore, and Strasbourg – are illustrative. In the first two the engines were at idle and the crew were unaware that the power was being commanded to idle by the Auto-thrust. This information is clearly presented at the left end of the Flight Mode Annunciator or FMA, which appears in a band across the top of the Primary Flight Display, or PFD. The Auto-thrust Mode is what the Auto-thrust thinks it is doing. The only acceptable modes for approach are Speed and Off. At Mulhouse and Bangalore the Auto-thrust Mode was reading Idle.
What was not clearly understood at the time of these crashes was how to change the Auto-thrust mode from Idle to Speed, which is to turn off both Flight Directors. Further, the annunciation of the Flight Director modes on the FMA was not as communicative as it is today. At Bangalore one pilot turned off his Flight Director and the other did not. As a result the Auto-thrust mode remained in Idle. Today in that situation the FMA would show 1FD- , meaning that FD 1 is operating on the left side and that FD2, on the right side, is off. (With both FD’s on the FMA would show 1FD2). Both pilots can see what is going on. This improvement was implemented after analysis of these crashes.
The Strasbourg crash resulted in another improvement in the airplane-pilot interface. The flight was performing a non-precision approach which specified a Flight Path Angle. The crew selected -3.3 into the Flight Control Unit but failed to switch it to Track/Flight Path Angle mode. The FCU remained in Heading/Vertical Speed mode and interpreted the command as -3300 feet per minute. (There is a big difference between the two. At normal approach speeds a Flight Path Angle of -3.3 would be 800-900 fpm.) The presentation has since been changed in two important ways: first, to change the HDG/VS mode on the FCU to show 3300 while leaving the TRK/FPA mode as 3.3. Second, the commanded rates are now repeated on the Flight Mode Annunciator.
In these accidents the crew were not aware of what the software was doing. In the following example, the loss of an A330 in flight test at Toulouse in 1994, the crew were not aware of a crucial software limitation.
In most autopilots there is an altitude capture mode. In Airbus aircraft this is known as ALT*, or “Alt Star.” The computer uses the selected altitude and the vertical speed to calculate how far ahead to begin the capture maneuver, which is an asymptotic curve. Higher vertical speeds require that the maneuver be begun earlier if “G” forces are to remain within limits. Crucially, because the software calculates the curve based vertical speed, it de facto assumes that the thrust available at the start of the capture maneuver will remain available. Thus the loss of an engine while in ALT* is a first-rate emergency requiring flight crew intervention within a few seconds.
Man/Machine Communication
I present these examples not as an exhaustive course on Airbus software, but as an illustration of how extra intelligence brings with it extra complication. First, the communication between man and machine is of paramount importance. The interface cannot be too well-designed and the pilot cannot take too much care in maintaining effective two-way communication. This is why at my airline any change in the FMA was verbalized by the Pilot Flying, in effect giving voice to the machine and keeping the three pilots (two human, one cybernetic) on the same page.
Second, each time a task is assigned to automation the process must remain transparent to the pilot. He must understand in general terms what the computers are doing, and even more importantly what they are not doing. Should the automation for any reason drop the task it must be immediately obvious to the pilot and he must have steps rehearsed which let him take control and do the task himself.
Engine failure in ALT* is a good example. With today's improved FCU interface the pilot can push the Vertical Speed knob, which simultaneously selects V/S as the vertical mode and sets the target V/S to zero. In less than a second he has intervened, taken control, and given himself time.
If altitude cannot be maintained on the remaining engine(s) he can twist the knob to set a modest descent. Then the drill calls for getting a clearance to a lower altitude, turning off the Auto-thrust and setting Maximum Continuous Thrust on the good engine(s), selecting the cleared altitude and Pulling the Altitude knob to select Open Descent. Speed and thrust can then be adjusted to suit the situation.
The above procedure is not difficult, is easily performed in the time available before losing control, and requires no particular skill. What it does require of the pilot is that he view the airplane (and her wonders of automation) as an equal: a skilled pilot who nevertheless can have a bad day, make a mistake, or be simply unavailable.
Anthropomorphism
I know I am not alone in assigning a personality to the Airbus. I have said elsewhere that I came to regard her as a friend, or more than a friend. I (ahem) even loved her. Perhaps I still do and that why I am writing this.
Wait, though. I know full well she is aluminum, carbon fibre, and Intel and Motorola Assembler. I also know she is a damn good pilot and that she can be trusted like a close friend. But – and this is the important part – she is my equal. I can fly too, but I sometimes make mistakes, have a bad day, or fail to communicate effectively. Ditto my software friend. I can be blinded by pride. Ditto my software friend. She is French and she has pride in her DNA.
In the Simulator we practice Pilot Incapacitation, recognizing that to err is human. What we have a harder time with is Automation Incapacitation. This is perhaps a symptom of our reverence for something that is beyond our understanding, for our unrecognized assumption that technology is perfect, or at least better than we are. This unrecognized and unwarranted assumption can be fatal.
It is much better to appreciate her as an equal and deal with her as a whole person, warts and all.
Feedback and Feel
Let's dig a step further. I believe what pilots are talking about, when they say Airbus aircraft are video games, is the lack of feedback and feel in the controls. The throttles, for example, do not move when the Auto-thrust is active. The pilot sees only Speed on the FMA and the engine indications on the ECAM. To take control smoothly (for example to do a manual approach) he must pull the thrust levers back until the little green donuts match the current thrust, and click the off button on the lever. The FMA says Off and he's on his own. But the approach is still a bit of a parlor trick because there is no feel in the sidestick. When a conventional aircraft gets slow increasing back pressure is necessary to keep the nose from dropping. Not so in an Airbus. Instead, the Autotrim will move the stabilizer nose-up to maintain 1G flight. The pilot's eye has to dart to the airspeed indicator to get what he might have sensed in the stick or control column. All of this contributes to the “video game” feel.
Perhaps a direct Angle of Attack readout in a Heads Up Display would compensate for the lack of feel. But this is ignoring an essential fact: the Airbus is a conventional airframe, with positive aerodynamic longitudinal stability. It is not like some fighter aircraft with neutral or negative longitudinal stability, where the aircraft is uncontrollable without fly-by-wire. The stability is there, but it is shielded from the pilot.
It must be pointed out that the Airbus is a beautiful airplane and a joy to fly and that it has hundreds of wonderful design features I would not like to see disappear. Just one example is “the hook” (the display on the airspeed tape of Vls (lowest selectable speed)) and its relationship to “the bug” (Vapp, or final approach speed). The bug speed is calculated by the FMCG (Flight Management and Guidance Computer) based on the Gross Weight (or Zero Fuel Weight) entered by the pilots. The hook is calculated from first principles by comparing Angle of Attack with dynamic pressure (airspeed). In a normal approach these are 0.5 cm (1/4 inch) apart. This is one of those comfort crosschecks for pilots. If the bug and the hook are too close together, the weight entered in the FMCD is likely wrong, and the calculated Vapp is too slow.
But even here an intelligence has been interposed between the pilot and his aircraft. Why not also display the Angle of Attack directly, and always fly the approach at the same angle of attack regardless of weight? (See my blog AF 447 – Let's Talk about Why – 1: Angle of Attack). It is this interposition of intelligence that contributes to what I see as the problem: the illusion of Virtual Reality.
Virtual Reality
Flying an airplane, any airplane, is a very real job. The airplane can be a bear or sweet to fly, it can be automated or not, it can “land itself.” But the bottom line of the captain's job does not change, and that is to be the arbiter of last resort: the man or woman who imagines, constructs, and sees the picture that determines the outcome. It is his or her job to maintain that picture. In the trade we call it situational awareness. If something goes wrong and that picture is wrong people die. And if the captain believes the glass display before him is superior to his own mental image, then he will be more likely to abdicate his responsibility to maintain situational awareness.
Today's glass cockpit is seductive. A wealth of information sits before the pilot: some of it is raw data; often it has been extensively processed into a colourful and sometimes beautiful picture. Like a video game, this is virtual reality. Software is doing the imagining for the pilot.
It can be argued that the picture in the pilot's head is also virtual reality, merely a representation of the external world. But this argument does not acknowledge the survival instinct that guides the pilot's doubt and questioning, his constant checking for consistency, his testing of the obvious.
Airbus aircraft are beautiful and a joy to fly. But they are not perfect. Like all of us, they have a fatal flaw. The Ancient Greeks knew this hamartia as an essential component of human character. Bernard Ziegler, the brilliant designer of the Airbus software, has been quoted as saying he wanted to make the airplane pilot-proof. Consequentially, as I have shown, there are areas where the pilots have been shielded from useful, even essential, information. The Airbus pilot must work hard to ensure he is not entirely removed from the loop.
Reality for an airline passenger is not virtual. This game cannot be started over. The next time you hear someone say, this airplane lands itself, will you be comforted? Or will you be hoping that the pilots are not just along for the ride?
Thanks to the work of David Learmount at Flight Global, and that of the Wood's Hole Oceanographic Institution and the Bureau d’Enquêtes et d’Analyses, enough is now known about this accident to start looking for useful lessons and to analyze the data along with the BEA. Flight safety and the future of the piloting profession depend on this becoming a wide and serious conversation.
Pilots obsess about accidents for good reason. There is always so much to learn. The AF447 tragedy is an epochal example.
There is a mind-boggling number of lessons to be learned here, in a host of areas and disciplines: Pilot Training, Standard Operating Procedures, Instrument Flight, and Aircraft Design are but a few of them.
I will commit today to joining the conversation. I begin with a consideration of Angle of Attack.
Angle of Attack
Wolfgang Langewiesche (father of William) emphasized Angle of Attack in his excellent Stick and Rudder, published in 1944. Advocacy of AoA was an uphill battle then and it still is today. Instead of talking about AoA, we prefer to use airspeed and explain why certain speeds we use change with aircraft weight and G loading. Many or even most aircraft flying today have no Angle of Attack indication. The accident aircraft had two AoA sensors. The flight recorders had access to the signals from these sensors but the pilots did not, at least not when they needed it most.
Lift is produced when the air flowing over the top of a wing has a longer distance to travel than the air flowing underneath. The air “stretching out” over the top produces a lower pressure, allowing the higher pressure underneath to push the wing up. There is a caveat, however. The airflow must remain attached to the upper surface of the wing.
Imagine a cross-section of wing, with a line drawn from the middle of the rounded leading edge to the pointed trailing edge. This is the chord line. Now imagine an arrow pointing at the leading edge. This is the airflow.
If the arrow meets a (symmetrical) wing head-on there will be no lift. But let the wing meet the air at a slight angle and the airflow around the wing will no longer be symmetrical: it will meet the rounded leading edge at an angle and it will divide lower on the curve of the leading edge. The air flowing over the wing will have a longer distance to travel. Lift will be produced.
The angle at which the airflow meets the chord line is called the Angle of Attack. Up to a point, increasing the Angle of Attack will increase lift. But beyond a certain point – usually about 16° – lift will instead decrease because the airflow is beginning to separate from the upper surface of the wing. This is called the aerodynamic stall, and it always happens at the same Angle of Attack.
Angle of Attack is controlled by the elevators, the control surfaces on the trailing edge of the horizontal tail. When the pilot pulls back on the stick, the elevators lift, causing a down-force on the tail and forcing the wing to meet the air at a higher Angle of Attack. Trim tabs (small surfaces at the trailing edge of the elevators) can be moved to change the neutral position of the stick. (Another way to think of it is the trim tabs change the Angle of Attack at which there is zero stick force.)
In a modern jet transport the entire horizontal tail is usually moveable. This is because of the very wide speed range of the jet and because flaps and leading edge slats also change the “trim.” The other side of the coin is that this horizontal tail, or stabilizer, is very powerful in modern jet transports. A runaway stabilizer is a true emergency. Traditionally there has been a STAB IN MOTION aural warning, and an emergency cutout switch close to hand. Most cases where the stabilizer ran all the way up or down in flight have resulted in the loss of all on board.
In most aircraft the pilot is used to trimming as he flies. A change of speed or configuration, be it in a Beech Bonanza or a DC-9, will require a trim change. With some experience on type the pilot knows (for example on a DC-9) that extending the leading edge slats will require two beeps (of the STAB IN MOTION aural warning) of nose-up trim. He can use the thumb switches on the yoke to move the stabilizer as the slats are extending and thus remain stick-neutral during the configuration change. This is part of anticipation, or staying ahead of the aircraft.
Airbus aircraft, from the A320 onwards, are different. They are fly by wire, where computers are interposed between the pilots' sidestick inputs and the control surfaces. This arrangement allows some elegant additions to aircraft design, such as envelope protection (which among other things makes it impossible for the pilot to stall the aircraft) and, relevant to our discussion today, stick force per G and autotrim.
In Normal Law, which is where the Airbus is most (and the pilot hopes, all) of the time, configuration changes can be made hands off, even flying by hand. Of course the pilot has the tips of his fingers on the sidestick, but he can make a configuration change with no pitch input because the control system, in Normal Law, will maintain 1G flight. When he calls for FLAP 1 and the leading edge slats extend, the nose-down pitch is sensed and countered by the system, maintaining 1G flight. (1G is what you experience sitting in a chair at home or in an aircraft at cruise in smooth air). In effect, the airplane is doing the anticipation for the pilot.
Like the transition in the late 1950's from props to jets, fly-by-wire has been a major change for pilots. In general we welcome it for the many advantages it offers.
Experience has shown that to do his job, which is to ensure the safe arrival of his aircraft, the pilot must fully understand a much more complex airplane. Chesley Sullenberger reached up and started the APU (the Auxiliary Power Unit, a small turbine in the tail which can supply electrical and hydraulic power on the ground or in flight) as soon as his engines lost power. Why? Because he knew his airplane and he knew he wanted to keep it in Normal Law until touchdown.
The transition from props to jets was all about speed range, speed brakes and spoilers, high Mach number, coffin corner, Dutch Roll and super-stall, but in everyday life it was more about high drag on approach, no propwash, slow spool-up times, and operating on the back side of the power curve. This change took some adjustment on the part of pilots: the more experienced pilots had more adjustments to make. The same is true with the transition to fly-by-wire.
In a traditional airplane the pilot controls Angle of Attack with the elevator and the trim tabs or stabilizer. (More often he will be thinking of Airspeed, which is the constant-weight, 1G manifestation of AofA). He is used to feel, which is essentially the change in elevator neutral point with AofA. Should the aircraft slow on approach, the nose will get “heavy”, prompting him to pull back or trim nose-up.
That feel is totally absent in Airbus aircraft. (Boeing, in the B777, have added artificial feel to their fly-by-wire system). The Airbus pilot points and shoots, so to speak. Flying by hand he can take the bird, turning on a symbol (like a bird or an aircraft seen from behind) on his Primary Flight Display. The bird shows where his velocity vector is pointed; in other words, where is airplane will be so many seconds from now if he makes no further adjustments. On approach he can pin the bird on his flare point on the runway and either let the autothrust take care of the speed or adjust the thrust levers manually. If he does the latter, he must remember that there is no feel or feedback in the sidestick.
Obviously there are quite different assumptions operating during an approach in a Bonanza, one one hand, and an Airbus, on the other. This is not necessarily a bad thing. Take for example driving a car versus riding a motorcycle. In a car you steer with the steering wheel. In a motorcycle you counter-steer, putting pressure on the inside foot-peg and forward pressure on the inside bar, in effect trying to steer the front wheel the opposite way.
But you know you're on a motorcycle and not in a car. You have learned how to ride a motorcycle.
Consider, however, flying an Airbus if something goes wrong with a sensor or a computer and you wind up in Alternate Law or Direct Law. You are in the same vehicle but suddenly the rules have changed; the assumptions have changed. It is, in effect, no longer the same machine. This is a recipe which messes with a pilot's head.
Unfortunately, experience has shown that Direct Law, where control displacement is proportional to stick force and the airplane handles like a wet fish, is actually the more benign of the two degraded modes. There is a big message in red on the ECAM saying USE MAN PITCH TRIM. The pilot moves the THS (Trimmable Horizontal Stabilizer) by moving a wheel almost a foot in diameter. This is old-style, normal airplane flying, commanding AofA with stick force and trim. There is still no feel in the sidestick, but the procedure is familiar.
Alas, in Alternate Law there is no such familiarity. It is still point-and-shoot, sort of, but autotrim is still working. As long as there is back pressure on the stick the THS trims nose-up, and vice-versa. There is NO Stabilizer in Motion warning except the movement of the trim wheels. That would seem to be an easy thing to detect, but I can testify from personal experience that it is not. On every landing (in Normal Law) the flight control computers memorize the attitude at 50 feet Radio Altitude and at 30 feet start rolling in nose-down trim, in effect trying to mimic the feel of a normal aircraft slowing in the flare. In almost a decade of flying as Captain and Training Captain, whether as Pilot Flying or Pilot Not Flying, I cannot remember ever seeing the trim wheels move.
In two recent accidents an Airbus has hit the ocean with the THS wound to full nose up. In both cases the aircraft was in Alternate Law.
I am not an engineer. There are likely many ramifications that have not crossed my mind. But sitting here this afternoon my personal recommendation would be as follows:
Thanks to the work of David Learmount at Flight Global, and that of the Wood's Hole Oceanographic Institution and the Bureau d’Enquêtes et d’Analyses, enough is now known about this accident to start looking for useful lessons and to analyze the data along with the BEA. Flight safety and the future of the piloting profession depend on this becoming a wide and serious conversation.
Pilots obsess about accidents for good reason. There is always so much to learn. The AF447 tragedy is an epochal example.
There is a mind-boggling number of lessons to be learned here, in a host of areas and disciplines: Pilot Training, Standard Operating Procedures, Instrument Flight, and Aircraft Design are but a few of them.
I will commit today to joining the conversation. I begin with a consideration of Angle of Attack.
Angle of Attack
Wolfgang Langewiesche (father of William) emphasized Angle of Attack in his excellent Stick and Rudder, published in 1944. Advocacy of AoA was an uphill battle then and it still is today. Instead of talking about AoA, we prefer to use airspeed and explain why certain speeds we use change with aircraft weight and G loading. Many or even most aircraft flying today have no Angle of Attack indication. The accident aircraft had two AoA sensors. The flight recorders had access to the signals from these sensors but the pilots did not, at least not when they needed it most.
Lift is produced when the air flowing over the top of a wing has a longer distance to travel than the air flowing underneath. The air “stretching out” over the top produces a lower pressure, allowing the higher pressure underneath to push the wing up. There is a caveat, however. The airflow must remain attached to the upper surface of the wing.
Imagine a cross-section of wing, with a line drawn from the middle of the rounded leading edge to the pointed trailing edge. This is the chord line. Now imagine an arrow pointing at the leading edge. This is the airflow.
If the arrow meets a (symmetrical) wing head-on there will be no lift. But let the wing meet the air at a slight angle and the airflow around the wing will no longer be symmetrical: it will meet the rounded leading edge at an angle and it will divide lower on the curve of the leading edge. The air flowing over the wing will have a longer distance to travel. Lift will be produced.
The angle at which the airflow meets the chord line is called the Angle of Attack. Up to a point, increasing the Angle of Attack will increase lift. But beyond a certain point – usually about 16° – lift will instead decrease because the airflow is beginning to separate from the upper surface of the wing. This is called the aerodynamic stall, and it always happens at the same Angle of Attack.
Angle of Attack is controlled by the elevators, the control surfaces on the trailing edge of the horizontal tail. When the pilot pulls back on the stick, the elevators lift, causing a down-force on the tail and forcing the wing to meet the air at a higher Angle of Attack. Trim tabs (small surfaces at the trailing edge of the elevators) can be moved to change the neutral position of the stick. (Another way to think of it is the trim tabs change the Angle of Attack at which there is zero stick force.)
In a modern jet transport the entire horizontal tail is usually moveable. This is because of the very wide speed range of the jet and because flaps and leading edge slats also change the “trim.” The other side of the coin is that this horizontal tail, or stabilizer, is very powerful in modern jet transports. A runaway stabilizer is a true emergency. Traditionally there has been a STAB IN MOTION aural warning, and an emergency cutout switch close to hand. Most cases where the stabilizer ran all the way up or down in flight have resulted in the loss of all on board.
In most aircraft the pilot is used to trimming as he flies. A change of speed or configuration, be it in a Beech Bonanza or a DC-9, will require a trim change. With some experience on type the pilot knows (for example on a DC-9) that extending the leading edge slats will require two beeps (of the STAB IN MOTION aural warning) of nose-up trim. He can use the thumb switches on the yoke to move the stabilizer as the slats are extending and thus remain stick-neutral during the configuration change. This is part of anticipation, or staying ahead of the aircraft.
Airbus aircraft, from the A320 onwards, are different. They are fly by wire, where computers are interposed between the pilots' sidestick inputs and the control surfaces. This arrangement allows some elegant additions to aircraft design, such as envelope protection (which among other things makes it impossible for the pilot to stall the aircraft) and, relevant to our discussion today, stick force per G and autotrim.
In Normal Law, which is where the Airbus is most (and the pilot hopes, all) of the time, configuration changes can be made hands off, even flying by hand. Of course the pilot has the tips of his fingers on the sidestick, but he can make a configuration change with no pitch input because the control system, in Normal Law, will maintain 1G flight. When he calls for FLAP 1 and the leading edge slats extend, the nose-down pitch is sensed and countered by the system, maintaining 1G flight. (1G is what you experience sitting in a chair at home or in an aircraft at cruise in smooth air). In effect, the airplane is doing the anticipation for the pilot.
Like the transition in the late 1950's from props to jets, fly-by-wire has been a major change for pilots. In general we welcome it for the many advantages it offers.
Experience has shown that to do his job, which is to ensure the safe arrival of his aircraft, the pilot must fully understand a much more complex airplane. Chesley Sullenberger reached up and started the APU (the Auxiliary Power Unit, a small turbine in the tail which can supply electrical and hydraulic power on the ground or in flight) as soon as his engines lost power. Why? Because he knew his airplane and he knew he wanted to keep it in Normal Law until touchdown.
The transition from props to jets was all about speed range, speed brakes and spoilers, high Mach number, coffin corner, Dutch Roll and super-stall, but in everyday life it was more about high drag on approach, no propwash, slow spool-up times, and operating on the back side of the power curve. This change took some adjustment on the part of pilots: the more experienced pilots had more adjustments to make. The same is true with the transition to fly-by-wire.
In a traditional airplane the pilot controls Angle of Attack with the elevator and the trim tabs or stabilizer. (More often he will be thinking of Airspeed, which is the constant-weight, 1G manifestation of AofA). He is used to feel, which is essentially the change in elevator neutral point with AofA. Should the aircraft slow on approach, the nose will get “heavy”, prompting him to pull back or trim nose-up.
That feel is totally absent in Airbus aircraft. (Boeing, in the B777, have added artificial feel to their fly-by-wire system). The Airbus pilot points and shoots, so to speak. Flying by hand he can take the bird, turning on a symbol (like a bird or an aircraft seen from behind) on his Primary Flight Display. The bird shows where his velocity vector is pointed; in other words, where is airplane will be so many seconds from now if he makes no further adjustments. On approach he can pin the bird on his flare point on the runway and either let the autothrust take care of the speed or adjust the thrust levers manually. If he does the latter, he must remember that there is no feel or feedback in the sidestick.
Obviously there are quite different assumptions operating during an approach in a Bonanza, one one hand, and an Airbus, on the other. This is not necessarily a bad thing. Take for example driving a car versus riding a motorcycle. In a car you steer with the steering wheel. In a motorcycle you counter-steer, putting pressure on the inside foot-peg and forward pressure on the inside bar, in effect trying to steer the front wheel the opposite way.
But you know you're on a motorcycle and not in a car. You have learned how to ride a motorcycle.
Consider, however, flying an Airbus if something goes wrong with a sensor or a computer and you wind up in Alternate Law or Direct Law. You are in the same vehicle but suddenly the rules have changed; the assumptions have changed. It is, in effect, no longer the same machine. This is a recipe which messes with a pilot's head.
Unfortunately, experience has shown that Direct Law, where control displacement is proportional to stick force and the airplane handles like a wet fish, is actually the more benign of the two degraded modes. There is a big message in red on the ECAM saying USE MAN PITCH TRIM. The pilot moves the THS (Trimmable Horizontal Stabilizer) by moving a wheel almost a foot in diameter. This is old-style, normal airplane flying, commanding AofA with stick force and trim. There is still no feel in the sidestick, but the procedure is familiar.
Alas, in Alternate Law there is no such familiarity. It is still point-and-shoot, sort of, but autotrim is still working. As long as there is back pressure on the stick the THS trims nose-up, and vice-versa. There is NO Stabilizer in Motion warning except the movement of the trim wheels. That would seem to be an easy thing to detect, but I can testify from personal experience that it is not. On every landing (in Normal Law) the flight control computers memorize the attitude at 50 feet Radio Altitude and at 30 feet start rolling in nose-down trim, in effect trying to mimic the feel of a normal aircraft slowing in the flare. In almost a decade of flying as Captain and Training Captain, whether as Pilot Flying or Pilot Not Flying, I cannot remember ever seeing the trim wheels move.
In two recent accidents an Airbus has hit the ocean with the THS wound to full nose up. In both cases the aircraft was in Alternate Law.
I am not an engineer. There are likely many ramifications that have not crossed my mind. But sitting here this afternoon my personal recommendation would be as follows:
The treasure in the tragedy of AF 447 is what it can teach us.
Thank you, David Learmount at Flight Global, for calling for dialogue. Thank you Wood's Hole Oceanographic Institution and Bureau d’Enquêtes et d’Analyses for recovering data we can all work from.
Thank you, David, for offering to share the blame. All of us share the blame, all the way down to the passenger who wants a cheaper flight. But in the end, blame is irrelevant. Blame only serves economic interests which demand simple answers. These answers will not be simple.
Let us all be brave and try to speak truth as we see it. Let us all bring our training and experience to bear on the recovered data and mine it for lessons and solutions.
The crew had not clearly declared who was flying the aircraft – in airline lingo who was PF and who was PNF.
Airbus sidesticks add algebraically. When the aircraft hit the water the left sidestick had some nose-down input and the right sidestick was at full nose-up. The Trimmable Horizonal Stabilizer (the horizontal tail) was already at full nose-up, most likely because of continuous nose-up sidestick applied while in Alternate Law.
We’ll start with a new image today – the megaphone. Put the small end at the touchdown point, line it up with the runway, and tilt it up three degrees. This is the ILS, or at least a useful image of it.
The picture helps because it gives an instinctive feeling for what we have to do to fly an ILS:
Maneuver into the big end of the cone
Fly down its axis
Make smaller corrections as we get closer to the runway
Last time we talked about how to stay on the localizer – maintain the published track – and how we were using integration. Looking closer, we can take the integration back several levels:
A shallow turn for a short time means a small heading change, changing the track. Imagine the new track drawing an arrow – this is your velocity vector. The longer you stay on the track, the longer the arrow. Visualize (I'll add diagrams when I learn the software) the arrow: if you are correcting back to the on-course you'll want to return to your tracking heading when the tip of the arrow gets there.
The same method – integrate and visualize – works for the vertical axis:
Power + Pitch --> Vertical Speed
Use V/S as you would track to manage vertical displacement – to track the glideslope, if you will. The same method works in both axes:
Before you start the approach, have targets in mind – the published track, and a target vertical speed you calculate from your planned airspeed on approach: airspeed/2 X 10 = 600 fpm for 120 knots (if you have GPS, use your groundspeed).
Fly into the big end of the cone and center the localizer.
Fly the target heading and see what happens. Now you know something about the wind. Adjust your target. (If you have GPS, flying the published track will keep you on the localizer.)
Correct back on, then fly the new target. Repeat and get it nailed (at least for this altitude).
As the glideslope comes down to meet you, do what you need to get your target V/S. (It should be as little as possible and preferably only one thing: reduce RPM or MP a certain amount; put the gear down.)
See what happens. Adjust your target. (If you have GPS, glance at the groundspeed. If it's only 100 knots, your new target is 500 fpm.)
Correct back onto the glideslope by adjusting V/S, visualizing the arrow (your velocity vector in the vertical axis) intercepting the G/S. When you're back on, fly the new target.
Continue as above, visualizing the megaphone as it gets smaller, guiding you to that window 200 feet above the approach lights. (Your corrections are getting smaller and smaller.)
KEEP YOUR TARGETS IN YOUR HEAD RIGHT DOWN TO MINIMUMS. (They are now accurate to a degree or two of heading and 50-100 fpm.)
That's it! Simple, right?
Actually, it is, and it works, but it does take some thinking about. For example: if the needles are centered, are you flying down the axis of the megaphone?
Maintain the published track and you’ll stay on the localizer.
Sounds simple. Makes sense. But it’s not instinctive. You have to think about it.
Here’s a thought experiment. You are running a train down a straight track. You can’t see outside. You have a stopwatch, a remote paintgun, and an accurate speedometer. Your task is to make two marks on the track a mile apart.
Simple, right? You accelerate to 60 mph, hit the paintgun remote and the stopwatch at the same time. Exactly 60 seconds later you hit the paintgun remote again. Mission accomplished!
But what if you are flying an airplane doing 120 knots? You are on (over) the track and you hit the paintgun. You wait 30 seconds and hit it again. Where is the second blob of paint? Sure, it's 1 nautical mile ahead, but is it on the track?
Yes, if your track hasn't changed. That's easy for a train but a big IF for an airplane. The wind could change. Your heading could change. Then the second blob of paint will not be on the track. It will be off to one side. Your localizer needle will be off to one side.
In math this is an example of integration. You are adding up what happens to your position as a result of your velocity vector. The INS or IRS in an airliner does it. Experienced pilots do something like it in their heads.
If you have a Garmin 430 in your airplane you can go to NAV page 1 and fly so TRK is the same as DTK. If you don't you'll have to do it the old-fashioned way, flying heading to compensate for drift. Either way, try to have the picture in your head.
We'll speak more about integration in future blogs. It's a great help if you want to fly IFR with precision.
